Stop dyntopo updates from happening in masked areas

Fixes [#34348] Collapse short edges affects masked geometry
/projects.blender.org/tracker/?func=detail&aid=34348&group_id=9&atid=498

Fixed by disallowing topology updates on edges with a vertex masked at
50% or greater. This is a necessarily arbitrary choice; can't do
halfway updates for topology like we can with displacement.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55344 954f8c5b-7b00-dc11-b283-0030488c597c